ALMA Rewrites History of Universe's Stellar Baby Boom

Figure 1: Montage, combining data from ALMA with images from the NASA/ESA Hubble Space Telescope, for four distant galaxies. The ALMA images, represented in red, show the distant, background galaxies, being distorted by the gravitational lens effect produced by the galaxies in the foreground, depicted in the Hubble data in blue. The background galaxies appear warped into rings of light known as Einstein rings, which encircle the foreground galaxies.
Credit: ALMA (ESO/NRAO/NAOJ), Y. Hezaveh et al

Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) show that the most vigorous bursts of star birth in the cosmos took place much earlier than previously thought. The results are published in a set of three papers to appear in the journal Nature on 14 March 2013, and in the Astrophysical Journal with one publication led by scientists from the Max Planck Institute for Radio Astronomy in Bonn. The research is the most recent example of the discoveries coming from the new international ALMA observatory, which celebrates its inauguration today.

The most intense bursts of star birth are thought to have occurred in the early Universe, in massive, bright galaxies. These starburst galaxies convert vast reservoirs of cosmic gas and dust into new stars at a furious pace -- many hundreds of times faster than in stately spiral galaxies like our own galaxy, the Milky Way. By looking far into space, at galaxies so distant that their light has taken many billions of years to reach us, astronomers can observe this busy period in the Universe's youth.

"The more distant the galaxy, the further back in time one is looking, so by measuring their distances we can piece together a timeline of how vigorously the Universe was making new stars at different stages of its 13.7 billion year history," said Joaquin Vieira (California Institute of Technology, USA), who led the team and is lead author of the paper in the journal Nature.

The international team of researchers first discovered these distant and enigmatic starburst galaxies with the US National Science Foundation's 10-metre South Pole Telescope (SPT) and then used ALMA to zoom in on them to explore the stellar baby boom in the young Universe. They were surprised to find that many of these distant dusty star-forming galaxies are even further away than expected. This means that, on average, their bursts of star birth took place 12 billion years ago, when the Universe was just under 2 billion years old -- a full billion years earlier than previously thought.

Two of these galaxies are the most distant of their kind ever seen -- so distant that their light began its journey when the Universe was only one billion years old. What's more, in one of these record-breakers, water is among the molecules detected, marking the most distant observations of water in the cosmos published to date.

The team used the unrivalled sensitivity of ALMA to capture light from 26 of these galaxies at wavelengths of around three millimetres. Light at characteristic wavelengths is produced by gas molecules in these galaxies, and the wavelengths are stretched by the expansion of the Universe over the billions of years that it takes the light to reach us. By measuring the stretched wavelengths, astronomers can calculate how long the light's journey has taken, and place each galaxy at the right point in cosmic history.

"ALMA's sensitivity and wide wavelength range mean we could make our measurements in just a few minutes per galaxy -- about one hundred times faster than before," said Axel Weiss (Max Planck Institute for Radio Astronomy in Bonn, Germany), who led the work to measure the distances to the galaxies. "Previously, a measurement like this would have been a laborious process of combining data from both visible-light and radio telescopes."

In the majority of cases, the ALMA observations alone could pinpoint the distances, but for a few galaxies the team combined the ALMA data with measurements from other telescopes, including the Atacama Pathfinder Experiment (APEX) and ESO's Very Large Telescope.

The astronomers were using only a partial array of 16 of ALMA's full complement of 66 giant antennas, as the observatory was still under construction at an altitude of 5000 meters on the remote Chajnantor Plateau in the Chilean Andes. When complete, ALMA will be even more sensitive, and will be able to detect even fainter galaxies. For now, astronomers targeted the brighter ones. They took advantage of a helping hand from nature, too: using gravitational lensing, an effect predicted by Einstein's general theory of relativity, where light from a distant galaxy is distorted by the gravitational influence of a nearer foreground galaxy, which acts like a lens and makes the distant source appear brighter.

To understand by precisely how much this gravitational lensing brightened the view of the galaxies, the team made sharper images of them using more ALMA observations at wavelengths of around 0.9 millimeters.

"These beautiful pictures from ALMA show the background galaxies warped into multiple arcs of light known as Einstein rings, which encircle the foreground galaxies," said Yashar Hezaveh (McGill University, Montreal, Canada), who led the study of the gravitational lensing. "We are using the massive amount of dark matter surrounding galaxies half-way across the Universe as a cosmic telescope to make even more distant galaxies appear bigger and brighter."

Analysis of the distortion reveals that some of the distant star-forming galaxies are as bright as 40 trillion (40 million million) Suns, and that gravitational lensing has magnified this by up to 22 times.

"Only a few gravitationally lensed galaxies have been found before at these submillimeter wavelengths, but now SPT and ALMA have uncovered dozens of them." said Carlos De Breuck (ESO), a member of the team. "This kind of science was previously done mostly at visible-light wavelengths with the Hubble Space Telescope, but our results show that ALMA is a very powerful new player in the field."

"This is a great example of astronomers from around the world collaborating to make an amazing discovery with a state-of-the-art facility," said team member Daniel Marrone (University of Arizona, USA). "This is just the beginning for ALMA and for the study of these starburst galaxies. Our next step is to study these objects in greater detail and figure out exactly how and why they are forming stars at such prodigious rates."

Atacama Large Millimeter/submillimeter Array (ALMA): an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

European Southern Observatory (ESO): the foremost intergovernmental astronomy organization in Europe and the world's most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-meter European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become "the world's biggest eye on the sky".

Max Planck Institute for Radio Astronomy (MPIfR): Radio and infrared astronomy are the main areas of research. The activities of the institute encompass the whole area of astronomical observations throughout the electromagnetic spectrum. Theoretical Astrophysics is an additional research field. To investigate the physics of stars, galaxies and the universe, the researchers look into subjects like stellar evolution, young stellar objects, stars at a late stage of their evolution, pulsars, the interstellar medium of the Milky Way and other galaxies, magnetic fields in the universe, radio galaxies, quasars and other active galaxies, dust and gas at cosmological distances, galaxies at early stages of the evolution of the universe, cosmic rays, high-energy particle physics as well as the theory of stellar evolution and active galactic nuclei. Scientists of the institute are employing some of the largest and most important astronomical observatories worldwide. The institute operates the 100-m diameter radio telescope and the first international station of the LOFAR, the Low Frequency Array, at its Effelsberg site near Bad Muenstereifel/Germany.

Atacama Pathfinder Experiment (APEX) is a collaboration between Max-Planck-Institut fuer Radioastronomie (MPIfR) at 50%, Onsala Space Observatory (OSO) at 23%, and the European Southern Observatory (ESO) at 27% to construct and operate a modified ALMA prototype antenna as a single dish on the high altitude site of Llano Chajnantor. The telescope was manufactured by VERTEX Antennentechnik in Duisburg, Germany. Operation of APEX at Chajnantor is entrusted to ESO. Additional observations for a number of galaxies within the project were made by APEX, the VLT, the Australia Telescope Compact Array (ATCA) and the Submillimeter Array (SMA).